function w = savesac(w, direc, fname)
   %SAVESAC  creates a SAC file from a waveform
   % savesac(waveform)   saves a waveform to a sac file in the
   % current directory, with a default filename as described below.
   %
   % savesac(waveform, directory)  saves the  sac file in the
   % provided directory, but with default filenames as described below.
   %
   % savesac(waveform, directory, filename)   saves a sac file
   % based upon the waveform(s).  The number of filenames (given as CELLS)
   % must equal the number of waveform objects to save.
   %
   % If filename is left out, a default filename will be generated for each
   % SAC file.  The default name will be
   %   YYYYMMDD_HHmmss.STATION.CHANNEL.NETWORK
   %   if the network does not exist in the waveform (in User defined field
   %   KNETWK, which is the standard SAC field for this), then network as
   %   defined in the filename becomes '__';
   %   For example:
   %    for waveform COLA(BHZ) starting 1/5/2007 04:03:21.32 ,AK network -->
   %       20070105_040321.COLA.BHZ.AK
   %    for waveform COLA(BHZ) starting 1/5/2007 04:03:21.32 ,no network -->
   %       20070105_040321.COLA.BHZ.__
   %
   % w = savesac(w...) will add history to the waveform showing into which
   % file the waveform was written.
   %
   % The waveform object is parsed out and placed into the sac header, with
   % the following equivalent fields set.  That is, the SAC field is replaced
   % with the waveform based values.
   % _waveform_ 	_sac_
   % STATION		KSTNM
   % CHANNEL		KCMPNM
   % FREQUENCY		1 / DELTA (ODELTA is not used)
   % START			NZYEAR, NZJDAY, NZHOUR, NZMIN, NZSEC, NZMSEC
   % UNITS			IDEP (but with caveats.  See SAC header info from llanl)
   %
   %
   % savesac uses routines developed by Mike Thorn and modified by yours
   % truly,  stored in the @waveform/private/ directory.
   %
   % For more information about the SAC header, and the interpretation of the
   % fields, check out Lawrence Livermore's web site.  Search for "seismic
   % sac" and you'll find it.
   %
   
   
   % VERSION: 1.1 of waveform objects
   % AUTHOR: Celso Reyes
   % LASTUPDATE: 9/2/2009
   
   if exist('fname') && ~iscell(fname),
      fname = {fname};
   end
   
   if nargin == 1
      %place file in current directory
      direc = pwd;
   end
   if nargin== 3
      %filename has been specified.
      if numel(w) ~= numel(fname) %but not one file per waveform...
         error('Waveform:savesac:badFilenameCount',...
            'Incorrect number of filenames (%d) for (%d) waveforms', ...
            numel(fname), numel(w));
      end
   else % nargin ~= 3
      %filenames are to be generated automatically.
      %Format: YYYYMMDD_HHmmss.STATION.CHANNEL.NETWORK
      for n=1:numel(w)
         [Y,M,D,h,m,s] = datevec(get(w(n),'start'));
         try
            Net = get(w(n),'KNETWK'); %get network from existing sac info.
         catch
             Net = '__';
         end
         if isempty(Net)
            Net = '__';
         end
         
         fname(n) = {sprintf('%04d%02d%02d_%02d%02d%02d.%s.%s.%s',...
            Y,M,D,h,m,fix(s), ...
            get(w(n),'station'),get(w(n),'channel'), Net)};
      end
   end
   
   %ensure we're working with cells for consistency
   if ~iscell(fname)
      fname = {fname};
   end
   
   disp('Warning: if dates have been changed within the waveform object,');
   disp('these changes will not be reflected within any SAC file picks');
   disp('That is, a change to the waveform starting time-- by subsetting,');
   disp('extracting, or by use of SET-- will only be reflected in NZHOUR,');
   disp('NZJDAY, NZMIN, NZMSEC, NZSEC, and NZYEAR');
   
   for n=1: numel(w) %fname is a cell array
      %first column of SAC is times, seocond is data. third will be header
      sacHeader = waveform2sacheader(w(n));
      outputFileName = fullfile(direc,fname{n});
      writesac(outputFileName,sacHeader, get(w,'data'));
   end
end